If the recruits accept the assignment, they are diverted from the academy and - to protect their identities - into a separate training program run by the NYPD's Intelligence Unit. They are given a cover name and cover story, and are assigned to follow leads about potential terror threats in or around the city.

The NYPD refuses to say how many of the undercover officers have been deployed or describe their tactics. But the 2006 trial of Shahawar Matin Siraj - convicted of plotting to blow up the Herald Square subway station in Manhattan - gave rare glimpses into how the department investigates Islamic extremists.

A 23-year-old undercover detective using his alias, Kamil Pasha, testified that he was drafted once the NYPD learned he was born in Bangladesh and could speak Arabic and Urdu. He said training lasted only a few weeks in 2002 before he got an apartment in Brooklyn and began attending a mosque there as a "walking camera" for his police handlers.

"I was told to act like a civilian - hang out in the neighborhood, gather information," he testified.

The witness recounted how he met Siraj at an Islamic bookstore near the mosque. He claimed that the defendant advocated jihad against the United States for its support of Israel and argued that targeting both countries for suicide bombings was justified.

Siraj declared in December 2002 that, "The mission was not complete on 9/11 because it did not hit Wall Street," the witness said.

Pressed on cross-examination about his own beliefs, the undercover officer expressed dismay at Siraj's brand of Islam.

"Where in Islam does it say you can blow up a train station?" he said.

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Associated Press writers David Porter and Samantha Henry in Newark, N.J., and Matt Apuzzo and Eileen Sullivan in Washington contributed to this report.